The invention relates to a process for a compacting porous structural member having a complicated shape and an optional size by encapsulation of the member with a material of the same type and which is capable of sintering, and subsequently subjecting the encapsulated member to hot-isostatic pressing.
Molded articles made of ceramic non-oxidic materials such as, for example, silicon nitride, silicon carbide, boron nitride or boron carbide have continued to come into wider use. Such molded articles or structural members are very porous and, thus, require treatment to reduce their porosity.
A known process for reducing the porosity of such molded articles, for example, involves hot-isostatic molding or hot-isostatic pressing. Since a gas is used in this process as the medium for transferring pressure, the porous ceramic bodies have to be encapsulated by a gas-tight capsule prior to compacting the same. Because of the high compacting temperatures of silicon nitride, silicon carbide, boron nitride and boron carbide from which such molded articles are usually made, glasses having high softening temperatures such as, for example, fused silica Vycor.sup.(R)* or Duran.sup.(R)** glass are almost exclusively employed as capsule materials. FNT *Reg. TM, Corning Glass Works, Corning, NY, USA FNT **Reg. TM, Jenaer Glaswerk Schott & Gen, Mainz, West Germany
However, various crucial disadvantages are inherent in the use of a glass as the capsule material. For example, reactions will occur between the glass envelope and the ceramic molded articles at the high compacting temperatures employed which are in excess of 1600.degree. C. In addition, the above-mentioned glasses all have very much smaller coefficients of thermal expansion than th ceramic molded articles. Consequently, upon cooling very high stresses occur which may result in complete destruction of the molded articles or structural members. Still further, great difficulties are involved in removing the glass capsule envelope from such molded articles after hot-isostatic molding.
In German Offenlegungsschrift No. DE-OS 30 47 237 there is described a process which avoids such drawbacks. The described process, porous bodies made of a ceramic material and having a complicated shape, such as, for example, turbine blades, are encompassed or coated with an inert pressing powder such as, for example, boron nitride powder, and then melt-encapsulated in quartz glass capsules prior to being subjected to hot-isostatic molding. The boron nitride powder prevents the capsule material from reacting with the material of the structural member so that upon cooling, no stresses will occur and after hot-isostatic pressing the capsule can be readily removed. The described process is well suited for manufacturing turbine blades.
However, when larger structural members such as, for example, turbo-supercharger rotors or monolithic turbine wheels are to be compacted, the glass capsules have to be more voluminous. In such cases, however, they become so expensive that economical manufacture of such structural members is no longer possible.
In the DE-OS No. 28 12 986, there is described a process in which a structural member made of silicon nitride, which contains a compacting aid, is employed. The porous article is coated with a silicon nitride skin having a thickness from 25 to 250 .mu.m. In a subsequent heating step, part of the compacting aid is supposed to diffuse from the molded article into the silicon nitride skin. It is intended thereby to cause the silicon nitride layer to be compacted by undergoing a further temperature treatment so that it can be subjected to a high pressure atmosphere without cracking. Several crucial drawbacks are inherent in this process. It is known from the literature (e.g., G. Wotting, Dissertation, Technical University, Berlin, 1983, page 9; Sallmang and Scholze, Die physikalischen und chemischen Grundlagen der Keramik, Springer-Vergag, Berlin, 1968) that the diffusion rates in silicon nitride and also in silicon carbide, boron nitride and boron carbide, are extremely low due to the high proportion of covalent bonding of these materials. It follows therefrom that the diffusion from the interior of the structural member to the outside thereof of the compacting aid in an amount sufficiently high so that the external skin will become tight-sealed by sintering which may take several hundred hours. Another drawback resides in the fact that the article to be compacted must itself contain a high proportion of the compacting aid. However, the compacting aids adversely affect the high-temperature properties of the respective materials, while it is precisely these high-temperature properties of silicon nitride, silicon carbide, boron nitride and boron carbide containing no compacting aid that render these materials so attractive for making high precision structural members. Another disadvantage resides in the fact that the silicon nitride skin, as applied, will become an integral component of the finished article. However, in the manufacture of high precision structural members, the requirements as to homogeneity of the silicon metal slip, accuracy of the casting procedure, maintenance of the conditions in the nitrideforming step and diffusion of the compacting aid are very difficult to achieve in the described process of DE-OS No. 28 12 986. Thus, molded articles having varying dimensions will be obtained.
There exists, therefore, a need to provide a process for encapsulating porous molded articles having complicated shapes and optional sizes, wherein the articles or structural members do not have to contain compacting aids, the coefficients of thermal expansion of molded articles and of the capsule materials are compatible with each other, and in which no reactions can occur between the capsule materials and the material of the structural members. The present invention fulfills this need.